Method for electrochemical coloring of aluminum and alloys

ABSTRACT

THE COLORING OF OXIDE LAYERS OF ALUMINUM AND ALLOYS OF ALUMINUM BY IMMERSION AS AN ELECTRODE IN AN AQUEOUS ACID BATH CONTAINING A METAL SALT IN SOLUTION AND PASSING THROUGH THE BATH AN ALTERNATING CURRENT SUPERIMPOSED BY A DIRECT CURRENT HAVING AN INTENSITY LESS THAN THE EFFECTIVE INTENSITY OF THE ALTERNATING CURRENT.

Nov. 28, 1972 J. PATRIE METHOD FOR ELECTROCHEMICAL COLORING OF ALUMINUM AND ALLOYS Filed Julv 15, 1970 .INVENTOR. do: B47015 United States Patent Cffice 3,704,209 Patented Nov. 28, 1972 3,704,209 METHOD FOR ELECTROCHEMICAL COLORING F ALUMINUM AND ALLOYS Jos Patric, Grenoble, France, assignor to Cegedur GP Filed July 15, 1970, Ser. No. 54,878 Claims priority, application France, July 16, 1969, 6924138 Int. Cl. C23b 9/02 US. Cl. 20435 N 12 Claims ABSTRACT OF THE DISCLOSURE The coloring of oxide layers of aluminum and alloys of aluminum by immersion as an electrode in an aqueous acid bath containing a metal salt in solution and passing through the bath an alternating current superimposed by a direct current having an intensity less than the effective intensity of the alternating current.

This invention relates to the coloring of surfaces of aluminum or alloys of aluminum by electrochemical means.

It is known that oxide layers produced by anodizing aluminum can be given a stable finish color by depositing metal pigments into the pores of the layers by alternating current electrolysis in an aqueous acid solution of metal salts. One of the electrodes is formed of the component to be colored, While the other electrode is an auxiliary electrode, which may be inert, i.e. for example of graphite, stainless steel or the dissolved metal that may consist of the same metal as the cation of the salt which is dissolved in the bath.

Light color finishes are obtained by short electrolysis but it is difficult to produce such colors in a uniform and reproducible manner.

Deep color finishes can be obtained by prolonged electrolysis. However, the slowness of the operation is a disadvantage from an economical point of view.

It is an object of this invention to produce aluminum and alloys of aluminum having variable and uniform reproducible colors on the surface and to provide a method and means for the preparation of same.

The present invention is distinguished by the fact that a direct current is superimposed upon the alternating current, with the intensity of the direct current being lower than the effective intensity of the alternating current. Depending upon the direction of the direct current, the color finish is either strengthened or weakened by comparison to that which would be obtained during the same period of time with alternating current alone.

By coloring with superimposed alternating and direct currents, it is possible, in a single operation, to obtain a whole range of light to dark color finishes, by defining the direction of the direct current on the one hand, and by regulating the intensity of the direct current relative to the alternating current on the other hand. In addition, there is no need for the direct current to be maintained entirely constant. Equally good results are obtained with rectified but unfiltered current.

When the object to be colored is connected to the positive pole of the direct current source, uniform and reproducible clear color finishes are readily obtained, as required for commercial operation.

By contrast, when the object to be colored is connected to the negative pole of the direct current source, high coloring rates are obtained which are of considerable economic advantage in the production of dark color finishes.

Unexpectedly, the process of this invention markedly reduces the dissolution of the electrode used as the auxiliary electrode, as distinguished from the rapid dissolution when alternating current is used alone, the latter of which has the disadvantage of enriching the bath with metal ions as well as changing the pH value.

Any electrical device enabling the electrolysis bath to be fed with superimposed alternating current can be used. The arrangement shown in the figure, which is given by way of illustration, but not by way of limitation, enables the alternating and direct components of the electrolysis installation to be precisely regulated.

The alternating current is provided by a variable ratio transformer 1 in which the primary winding is connected to the main line while the secondary winding comprisesa center tap. Each of the ends of the secondary winding is connected to a component or to a number of compo-.

nents to be treated. Accordingly, it is possible simultaneously to color two components or two rows of components.

Arranged between htese two electrodes, the inert or soluble counter electrode is connected to the center tap of the secondary winding through a diode bridge. The diode is fed by a variable ratio transformer 2. The two alternations of current produced in the secondary winding are rectified. It is not absolutely essential to provide a filter capacitor at the terminals of the diode bridge. The auxiliary electrode is placed between the two electrodes fed with alternating current. It delivers the rectified current, always in the same direction, to each of the aluminum electrodes. From the alternative point of view, it receives from the aluminum electrodes two currents out of phase by the sums of which cancel each other out. It is advisable to place the auxiliary electrode between the two aluminum electrodes to prevent any direct passage of alternating current between these two electrodes.

'Balancing and current-limiting rheostats and instruments for measuring the alternating current and direct current are included in the circuits. The diode bridge can :be switched by an inverter (not shown in the drawing) to determine the direction in which the direct current is flowing through the circuit. This system of elfectively individualized circuits enables the component of each of the two types of current, alternating and direct, to be regulated with precision.

The present invention is applicable to the coloring of aluminum and its alloys after anodization.

As in the conventional process, the components to be colored can be subjected to surface treatments, such as pickling or brightening, depending upon whether it is desirable for the finished components to have a dull or bright appearance, respectively.

The anodizing pre-treatment is conventional and can be carried out in a sulphuric acid or chromic acid bath, under well known conditions, which result in the formation of a porous layer of alumina having a thickness within the range of 1 to 50 microns.

The anodized components are colored while being suspended in an acidic electrolytic bath containing metal current. The treatment time depends on the current intensity and can vary from 10 seconds to 30 minutes, and preferably within the range of 1 to 10 minutes.

The components thus colored are sealed by approximately 30 minutes immersion in boiling water, which may optionally contain from 0.5 to 2 grams of nickel acetate per liter.

The following examples are given by way of illustration, but not by way of limitation, of the practice of this invention:

EXAMPLE 1 Two sheets of aluminum alloy, containing 0.6% of magnesium, which have been electrolytically brightened and anodized to a layer having a thickness of 20 microns in a sulphuric acid bath, are placed in the arrangement previously described in a bath containing 15 g./liter of copper sulphate and acidified with sulphuric acid to a pH of 1.2 and maintained at 20 C. The auxiliary electrode, which consists of copper, is placed between the sheets of aluminum and connected to the positive terminal of the rectified current source. An alternating current of 0.6 a. etfJdm. on which a direct current of 0.1 a./dm. is superimposed is then passed through for 4 minutes.

A dark red color finish is obtained which, after sealing in boiling water, is resistant both to corrosion and to ultra-violet rays.

EXAMPLE 2 Two sheets of 99.5% pure aluminum, which have been satinized and anodized to a layer having a thickness of 15 microns in a sulphuric acid bath, are placed in a bath of copper sulphate and sulphuric acid the same as that of Example 1 with a counter electrode of copper. An alternating current of 0.6 a. eff./dm. on which a direct current of 0.2 a./dm. is superimposed, is passed through for 4 minutes with the auxiliary electrode functioning as the anode.

A very dark red color finish is obtained which, after the components have been sealed, is resistant both to corrosion and to ultra-violet rays.

EXAMPLE 3 Two sheets of aluminum alloy, containing 6% of magnesium, which have been satinized and anodized to a layer having a thickness of 10 microns in a sulphuric aicd bath, are placed in a bath which is the same as that of Example 1 maintained at 20 C., with a stainless steel counter electrode. An alternating current of 0.6 a. elf/dm. on which a direct current of 0.3 a./dm. is superimposed is passed through for 4 minutes, the auxiliary electrode acting :as the anode.

A black color finish is obtained which, after the components have been scaled, is resistant to corrosion and to ultra-violet rays.

EXAMPLE 4 Two sheets of an aluminum alloy containing 99.5% of aluminum, which have been chemically brightened and anodized to a layer thickness of microns in a sulphuric acid bath, are placed in a bath identical with that of Example 1, maintained at C., with a stainless steel counter electrode. An alternating current of 0.6 a. etfJdm. on which a direct current of 0.3 a./dm. is superimposed is then passed through for 30 seconds, the auxiliary electrode acting as the cathode.

A very clear reddish-copper color finish is obtained which, after the components have been sealed, is resistant to corrosion and to ultra-violet rays.

EXAMPLE 5 Two pieces of sections of an aluminum alloy, containing 0.5% of silicon and 0.5% of magnesium, which have been mechanically polished and anodized to a layer thick ness of 18 microns in a sulphuric acid bath, are placed in a bath containing 100 g./liter of nickel sulphate, g./liter of boric acid and g./liter of ammonium sulphate, with the bath maintained at 20 C. and with a counter electrode of nickel. An alternating current of 0.4 a. elfJdm. on which a direct current of 0.1 a./dm. is superimposed is then passed through for 4 minutes, the auxiliary electrode acting as the cathode.

A clear bronze color finish is obtained which, after sealing in boiling water, is resistant both to corrosion and to ultra-violet rays.

EXAMPLE 6 Two pieces of sections of an aluminum alloy containing 0.5 of silicon and 0.5 of magnesium, which have been satinized and anodized to a layer thickness of 12 microns in a sulphuric acid bath, are placed in a bath identical with that of Example 5, maintained at 20 C., and with a counter electrode of nickel. An alternating current of 0.4 a. elf./dm. on which a direct current of 0.3 a./dm. is superimposed is then passed through for a period of 4 minutes, the auxiliary electrode acting as the anode.

A dark bronze color finish is obtained which, after sealing in boiling water, is resistant to corrosion and to ultra-violet rays.

EXAMPLE 7 Two plates of an aluminum alloy containing 0.6% of magnesium, which have been chemically brightened and anodized to a layer thickness of 8 microns in a bath containing 50 g./liter of chromic acid, maintained at 50 C., for 50 minutes as 50 volts, are placed in a sulphuric acidcopper sulphate bath the same as that of Example 1, maintained at 20 C., with a counter electrode of copper. An alternating current of 0.6 a. efL/dm. on which a direct current of 0.3 a./dm. is superimposed is then passed through for 2 minutes, the auxiliary electrode being the cathode.

A copper-pink color finish is obtained, which, after the plates have been sealed for 30 minutes in boiling distilled water, is resistant to corrosion and to ultra-violet rays.

It will be apparent from the foregoing that I have provided an improved means for producing uniform and selective colors in a commercial manner onto surfaces of aluminum or alloys of aluminum.

It will be understood that changes may be made in the details of formulation and operation without departing from the spirit of the invention, especially as defined in the following claims.

I claim:

1. In a process for coloring aluminum and alloys of aluminum provided with a porous oxide layer, the improvement comprising immersing the surface of an anodized aluminum or alloy of aluminum as an electrode in an electrolysis bath formed of an aqueous acid solution containing a dissolved salt of a metal selected from the group consisting of copper, nickel, cobalt, cadmium, silver and manganese, using a counter-electrode made of a metal of said salt or of stainless steel and passing an alternating current superimposed with a direct current through the bath, the DC/AC current density ratio being within the range of to /4.

2. The process as claimed in claim 1 in which the object to be colored is connected with the positive terminal of the direct current source.

3. The process as claimed in claim 1 in which the object to be colored is connected with the negative terminal of the direct current source.

4. The process as claimed in claim 1 in which the metal salt is present in the bath in an amount within the range of 5 to 200 g./ liter.

5. The process as claimed in claim 1 in which the metal salt is present in the bath in an amount within the range of 15 to g./liter.

6. The process as claimed in claim 1 in which the bath is maintained at a temperature within the range of 15 to 30 C. during coloring treatment.

7. The process as claimed in claim 1 in which the mm 12. The colored aluminum or alloy of aluminum prerent density of the alternating current is within the range pared by the process of claim 1. of 0.1 to 1 a./dm.

8. The process as claimed in claim 1 in which the elec- References Cited trolytic treatment for coloring is maintained for a time 5 UNITED STATES PATENTS within the range of 10 seconds to 30 minutes.

9. The process as claimed in claim 1 in which the time of treatment in the electrolytic bath is within the range of 1 to 10 minutes JOHN H. MACK, Primary Examiner 10. The process as claimed in claim 1 which includes 10 R. J. FAY, Assistant Examiner the step of sealing the coating by immersion in boiling water X.R.

11. The process as claimed in claim 10 in which the boiling water contains 0.5 to 2 g./liter of nickel acetate.

3,382,160 5/1968 Asada 20435 N 

